The in vivo biological significance of nitric oxide has received a great deal of interest in recent years. The NOS enzyme has been shown to exist in at least two forms which can be constitutve, for normal physiologic function, or inducible, as a response to an external stimulus. A target for drug design is the developemnt of potent and selective inhibitors of iNOS. The S-alkylisothioureas have been reported as extremely potent and competitive inhibitors of iNOS and ethylisothiouronium hydrochloride (EITU) has been suggested as a potential drug for the chronic inhibition of iNOS. In our hands, EITU demonstrated a factor of 8 in selectivity for iNOS compared to cNOS. We have synthesized and characterized 2-fluoro-ethylisothiouronium hydrochloride (FEITU), which showed comparable binding affinity and selectivity to iNOS. Radiolabeling with F-18 was achieved in 2 hours in 3-5% yield via alkylation of thiourea with [F-18]fluoroethyl triflate. As a comparason, the weaker and non-selective NOS inhibitor methylisothiouronium hydrochloride (MITU) was prepared and radiolabeled with C-11. Reaction of thiourea with [C-11]methyl iodide afforded [C-11]MITU in >50% yield. The biodistribution of each radiotracer in rats was examined. [F-18]FEITU showed greater brain uptake than [C-11]MITU. Both radiotracers had similar uptake in the liver and were cleared through the kidney. Stability studies indicated that both compounds were metabolized rapidly in vivo and in vitro, and that of [F-18]FEITU was much faster (75% metabolized at 10 min post-injection). These results were expected as these types of compounds are powerful guanidylating species, however PET can provide important pharmacokinetic data on these compounds. In conclusion, we have radiolabeled and completed initial evaluation of two new NOS inhibitors both in vivo and in vitro.